1. Technical Field
The present invention relates to an image forming apparatus that is preferably applied to a black-and white or color digital multifunctional machine equipped with copying functions, facsimile functions and printer functions and to a copier.
2. Description of Related Art
In recent years, there has come to be put to practical use a digital color copier that conducts color image forming based on color image data relating to red (R) color, green (G) color and blue (B) color acquired from colored document images. In the copier of this kind, image information of the document is read by a scanner, and color image data relating to image information of the document are acquired.
Further, a laser recording apparatus is mounted on the copier, and a laser beam emitted from a semiconductor laser light source is used for exposure scanning on a photoreceptor drum having thereon prescribed voltage to record images, depending on YMCK image data which are obtained by color-converting RGB image data acquired from a scanner into image data of yellow (Y) color, magenta (M) color, cyan (C) color and black (K) color. Images recorded on the photoreceptor drum are developed by each toner of each color, then, colors are superposed on an intermediate transfer body, for example, and each image is transferred onto a prescribed sheet from the intermediate transfer body, to be fixed. As a result, a color document image can be copied.
In a field of the color image forming apparatus of this kind, an apparatus wherein a color image can be formed on each of both sides of the sheet has been developed and is manufactured. Double-face forming functions are used for forming an image for a front cover on a sheet and for forming an image for a back cover on a sheet, when creating a booklet, for example. In many cases, a sheet that is thicker than a sheet mentioned in the text is used as a sheet for each of the front cover and back cover.
Sheets for the front cover and the back cover after double-face image forming are supposed to be subjected to post-processing such as center-folding and staple processing. In the double-face image forming processing of this kind, it is known that, after an image is formed on one side of a sheet, the sheet shrinks. The reason for this phenomenon is that a sheet onto which a color toner image has been transferred is subjected to thermal shrinkage by fixing processing, and the thicker the sheet is, the more remarkable the shrinkage is.
Each of FIGS. 17(A) and 17(B) is a diagram illustrating an example of shrinkage of sheet size in the case of double-face image forming. Sheet P shown in FIG. 17 (A) is in the state before fixing after being subjected to secondary transfer of color toner images. In sheet sizes for sheet P, a longitudinal length is L mm, and a lateral width is W mm. Sheet P′ shown in FIG. 17(B) is in the state after fixing of sheet P. In sheet sizes for sheet P′, a longitudinal length is constricted to L′ mm, and a lateral width is constricted to W′ mm. The reason for shrinkage of sheet sizes is considered to be moisture dessipation in the course of fixing. An image needs to be formed on the rear face of the sheet, taking such shrinkage of sheet sizes of sheet P into consideration. Incidentally, if image forming conditions are not adjusted to sheet sizes L′ mm×W′ mm after shrinkage, an image forming position (size) for the front face is deviated from that for the rear face.
A driving clock (hereinafter referred to as CLK) frequency of a polygon motor is changed, taking such shrinkage of sheet sizes of sheet P into consideration. When F0 represents polygon driving CLK frequency before shrinkage, namely, in the course of image forming on the front face, and F represents polygon driving CLK frequency after shrinkage, namely, in the course of image forming on the rear face, establishment is made so that F=F0×L/L′ may hold.
Further, pixel CLK frequency that controls a laser beam is changed. When f0 represents pixel CLK frequency before shrinkage and f represents pixel CLK frequency after shrinkage, establishment is made so that f=(L/L′)×(W/W′)×f0 may hold. By changing a polygon driving CLK frequency and a pixel CLK frequency, in consideration of shrinkage in sheet sizes for sheet P as stated above, it is possible to obtain images which are well-registered between the front face and the rear face.
Further, when the polygon driving CLK frequency is changed from F0 to F under the assumption that V0 represents a process linear speed before shrinkage, G0 represents a gap between processes before shrinkage, process gap G represents a distance between units and V represents a process linear speed, apparent process linear speed V is changed as shown below.    (1) Apparent process linear speed V=V0×F0/F=v0×L′/L    (2) Gap between processes G (pixel)=G0×V0/V=G0×L/L′In this case, the process linear speed V corresponds to a rotation speed of a photoreceptor representing an image carrier on which an image is formed.
Therefore, correction for an amount of front-face/rear-face magnification change (which is also called front-face/rear-face magnification correction or image size correction, after this) is needed even for a correction amount for color registration error which corresponds to gap between processes G. Accordingly, a polygon mirror which requires plane phase adjustment is subjected to practice of plane phase control when switching between front face and rear face. Control of the rotation speed of the polygon mirror and control of plane phase of a polygon mirror for each color of Y, M and C are practiced not only for double-face image forming processing but also for switching of trays.
For practicing image size correction in the case of switching between the front face and the rear face of a sheet or between trays, there is employed a method to control a rotation speed and a phase of a polygon mirror. Each of FIGS. 18(A)-18(I) is a time chart showing an example of image forming operations (for Y color) in the case of switching trays in an image writing unit for each of Y, M, C and K, relating to the conventional example.
A VTOP signal shown in FIG. 18(A) is a signal that rises in synchronization with an index signal (hereinafter referred to as KIDX signal) for forming K color images shown in FIG. 18(I), after a leading edge of the sheet fed out of tray 1 is detected by an unillustrated leading edge detection sensor. YVV start timing shown in FIG. 18(B) is for a signal that rises in synchronization with KIDX signal, where an unillustrated KIDX counter is started, and the number of pulses for KIDX signal is counted.
A YVV signal shown in FIG. 18(C) is a signal that rises in synchronization with an index signal (hereinafter referred to as YIDX signal) for forming Y color images shown in FIG. 18(D). During the period of “H” level of the YVV signal, an image in Y color is formed on a sheet coming from tray 1, and after completion of the foregoing, there is made control for changing a rotation speed of a polygon mirror for forming an image in Y color. In this case, a frequency of the YIDX signal is fluctuated-until the rotation speed of the polygon mirror is stabilized. With regard to the sheet for second page fed out of tray 2, image forming for Y color is started after waiting for stabilizing time Ty1 during which a rotation of the polygon mirror is stabilized.
In the same way, during the period of “H” level of the MVV signal shown in FIG. 18(E), an image in M color is formed on a sheet coming from tray 1, and after completion of the foregoing, there is made control for changing a rotation speed of a polygon mirror for forming-an image in M color. In this case, a frequency of the MIDX signal is fluctuated until the rotation speed of the polygon mirror for M color is stabilized. Phase change is controlled after waiting for stabilizing time Tm1 during which a rotation of the polygon mirror is stabilized. With regard to the sheet for second page fed out of tray 2, image forming for M color is started after waiting for stabilizing time Tm2 during which a rotation of the polygon mirror for M color is stabilized.
Further, during the period of “H” level of the CVV signal shown in FIG. 18(F), an image in C color is formed on a sheet coming from tray 1, and after completion of the foregoing, there is made control for changing a rotation speed of a polygon mirror for forming an image in C color. In this case, a frequency of the CIDX signal is fluctuated until the rotation speed of the polygon mirror for C color is stabilized. Phase change is controlled after waiting for stabilizing time Tc1 during which a rotation of the polygon mirror is stabilized. With regard to the sheet for second page fed out of tray 2, image forming for C color is started after waiting for stabilizing time Tc2 during which a rotation of the polygon mirror for C color is stabilized.
Further, KTV start timing shown in FIG. 18(G) is for a signal that rises in synchronization with KIDX signal, where an unillustrated KIDX counter is started, and the number of pulses for KIDX signal is counted KVV signal shown in FIG. 18(H) is a signal that rises in synchronization with KIDX signal shown in FIG. 18(I). During the period of “H” level of the KVV signal, an image in K color is formed on a sheet coming from tray 1, and after completion of the foregoing, there is made control for changing a rotation speed of a polygon mirror for forming an image in K color.
In this case, a frequency of the KIDX signal is fluctuated until the rotation speed of the polygon mirror for K color is stabilized. Phase change is controlled after waiting for stabilizing time Tk1 during which a rotation of the polygon mirror is stabilized. With regard to the sheet for second page fed out of tray 2, image forming for K color is started after waiting for stabilizing time Tk2 during which a rotation of the polygon mirror for K color is stabilized. In the example of image forming operations in the case of switching trays mentioned above, controls of rotation speed of polygon mirror for forming an image in each of Y, M and C colors and of a phase are practiced after the control of rotation speed of a polygon mirror for forming an image in K color has been completed, because it is carried out based on KIDX signals.
In association with the aforesaid control of a polygon mirror, a laser beam scanning apparatus is disclosed in Patent Document 1. In this laser beam scanning apparatus, there is provided a rotation phase calculating section that calculates a time difference between an optical beam detection signal corresponding to a reference polygon mirror and an optical beam detection signal [corresponding to a polygon mirror other than the reference polygon mirror, and compares phase control data based on the time difference with phase control data corresponding to a reference polygon mirror, to generate a rotation frequency. By providing such rotation phase calculating section, an orientation of the mirror surface of the polygon mirror can be controlled simply.
Patent Document 1: Unexamined Japanese Patent Application Publication NO. 9-230273 (FIG. 1 on page 5)
Incidentally, in the image forming apparatus applied by the inventors of the present invention, there is employed a method to correct magnifications for the front face and the rear face by changing rotation speed and phase of the polygon mirror by the use of pseudo index signals.
Each of FIGS. 19 (A)-18(O) is a time chart showing an example of operations (for Y color) in the case of correcting magnifications for the front face and the rear face of a color image forming apparatus.
A VTOP signal shown in FIG. 19(A) is a signal that rises in synchronization with YIDX signal shown in FIG. 19(F) after a leading edge of the sheet fed out of tray 1 is detected. YVV start timing shown in FIG. 19(D) is for a signal that rises in synchronization with YIDX signal, where an unillustrated YIDX counter is started, and the number of pulses for YIDX signal is counted. A YVV signal shown in FIG. 19(E) is a signal that rises in synchronization with YIDX signal shown in FIG. 19(F). During the period of “H” level of the YVV signal, an image in Y color is formed on a sheet coming from tray 1.
The control for changing a rotation speed of the polygon mirror for forming an image in Y color is carried out after completion of Y color image forming on the front face of the sheet, namely, after KVV signal shown in FIG. 19(H) has risen. In this case, a frequency of the YIDX signal is fluctuated until the rotation speed of the polygon mirror for Y color is stabilized. Phase change is controlled after waiting for stabilizing time Ty1′ during which a rotation of the polygon mirror is stabilized. With regard to the rear face of the sheet, image forming for Y color is started after waiting for stabilizing time Ty2′ during which a rotation of the polygon mirror for Y color is stabilized.
During the period of “H” level of the MVV signal shown in FIG. 19(H), an image in M color is formed on the front face of the sheet, and after completion of the foregoing, there is practiced a control for changing a rotation speed of a polygon mirror for forming an image in M color. In this case, a frequency of the MIDX signal is fluctuated until the rotation speed of the polygon mirror for M color is stabilized. Phase change is controlled after waiting for stabilizing time Tm1′ during which a rotation of the polygon mirror is stabilized. With regard to the rear face of the sheet, image forming for M color is started after waiting for stabilizing time Tm2′ during which a rotation of the polygon mirror for M color is stabilized.
During the period of “H” level of the CVV signal shown in FIG. 19(J), an image in C color is formed on the front face of the sheet, and after completion of the foregoing, there is practiced a control for changing a rotation speed of a polygon mirror for forming an image in C color. In this case, a frequency of the CIDX signal is fluctuated until the rotation speed of the polygon mirror for C color is stabilized. Phase change is controlled after waiting for stabilizing time Tc1′ during which a rotation of the polygon mirror is stabilized. With regard to the rear face of the sheet, image forming for C color is started after waiting for stabilizing time Tc2′ during which a rotation of the polygon mirror for C color is stabilized.
KVV start timing shown in FIG. 19(L) is for a signal that rises in synchronization with YIDX signal, where an unillustrated KIDX counter is started, and the number of pulses for YIDX signal is counted. KVV signal shown in FIG. 19(L) is a signal that rises in synchronization with KIDX signal shown in FIG. 19(M). During the period of “H” level of the KVV signal, an image in K color is formed on a sheet coming from tray 1, and after completion of the foregoing, there is made control for changing a rotation speed of a polygon mirror for forming an image in K color.
In this case, a frequency of the KIDX signal is fluctuated until the rotation speed of the polygon mirror for K color is stabilized. Phase change is controlled after waiting for stabilizing time Tk1′ during which a rotation of the polygon mirror is stabilized. With regard to the rear face of th sheet, image forming for K color is started after waiting for stabilizing time Tk2′ during which a rotation of the polygon mirror for K color is stabilized. Incidentally, T1 shown in FIG. 19(O) shows a period during which the start timing for each of YVV signal, MVV signal and CVV signal in the case of image forming on the front face is determined with MST-IDX1 serving as a count source, while T2 shows a period during which the start timing for each of YVV signal, MVV signal and CVV signal in the case of image forming on the rear face is determined with MST-IDX2 serving as a count source. By using pseudo index signals for correction of magnifications on the front face and the rear face as stated above, productivity is improved.
However, a color image forming apparatus relating to the conventional example has following problems.
(i) Phase changing control cannot be started until the moment when the polygon mirror arrives at its stable rotation by the instruction for changing rotation speed of the polygon mirror. Further, even after practicing the phase changing control, image forming processing cannot be started until the polygon mirror comes to its stable rotation. Therefore, when the magnification is corrected, the switching operation takes time, and productivity for double-face operations is lowered by conducting correction operation for magnifications.
In the example of image size correction in the case of tray switching shown in FIGS. 18(A)-18(I), it is not possible to start succeeding image formation processing for each color, without waiting for stabilizing time Ty1 for stabilizing polygon mirror rotation for Y-color, after Y-color image formation processing, without waiting for stabilizing time Tm1+Tm2 for stabilizing polygon mirror rotation for M-color, after M-color image formation processing, without waiting for stabilizing time Tc1+Tc2 for stabilizing polygon mirror rotation for C-color, after C-color image formation processing and without waiting for stabilizing time Tk1+Tk2 for stabilizing polygon mirror rotation for K-color, after K-color image formation processing. Therefore, high speed image formation processing is hampered by waiting for these stabilizing times Ty1, Tm1+Tm, Tc1+Tc2 and Tk1+Tk2.
(ii) The aforesaid problems are caused equally even in the case of switching image formation processing between the front face and rear face by using pseudo index signals shown in FIGS. 19(A)-19(O). In this case, it is not possible to start succeeding image formation processing for each color, without waiting for stabilizing time Ty1′+Ty2′ for stabilizing polygon mirror rotation for Y-color, after Y-color image formation processing, without waiting for stabilizing time Tm1′+Tm2′ for stabilizing polygon mirror rotation for M-color, after M-color image formation processing, without waiting for stabilizing time Tc1′+Tc2′ for stabilizing polygon mirror rotation for C-color, after C-color image formation processing and without waiting for stabilizing time Tk1′+Tk2′ for stabilizing polygon mirror rotation for K-color, after K-color image formation processing. Therefore, high speed image formation processing is hampered by waiting for these stabilizing times Ty1′+Ty2′, Tm1′+Tm2′, Tc1′+Tc2′ and Tk1′+Tk2′.(iii) In the laser beam apparatus seen in Patent Document 1, there is employed a method to generate polygon clock by comparing a counter cycle and a count value with a start-up point value calculated from a phase difference of detector pulse signals (index signals), concerning phase control of a polygon mirror. Even in this method, it is not possible to start color image forming processing for the succeeding page, without waiting stabilizing time after controlling a phase of a polygon mirror until its rotation is stabilized. Therefore, productivity in operations for image size correction is lowered, and continuous high speed processing for color images is prevented.
With the foregoing as a background, the invention has solved the aforesaid problems, and its objective is to provide an image forming apparatus wherein a decline of productivity in the course of correcting image size can be controlled, and continuous high speed processing for color images can be carried out.